A tunnel boring machine (“TBM”) is a tunnel excavation apparatus for constructing a tunnel or passageway through soil and rock strata. Typical conventional TBMs produce a smooth circular tunnel wall, typically with minimal collateral disturbance.
An early tunneling machine is disclosed in U.S. Pat. No. 17,650, to Wilson, and includes a large wheel with outboard scrapers and cutter wheels designed to bore an outer ring groove and a central cutting member that bores a small central hole. Wilson teaches exploding a charge of gunpowder in the central hole to detach rock intervening between the central hole and the ring groove.
A breakthrough that made TBMs efficient and reliable was the invention of the rotating head with rotatable cutter assemblies, developed by James S. Robbins, who later founded the Robbins Company. Initially, Robbins designed a TBM that used strong spikes that were mounted to a rotating cutterhead. However the TBM had the problem that the spikes would break frequently, resulting in expensive downtime. He discovered that by replacing these grinding spikes with longer lasting rotating cutter assemblies this problem was significantly reduced. Since then, successful modern TBMs have rotating cutter assemblies.
An early version of Robbins' rotating cutter TBM was able to cut 160 feet in 24 hours in shale, ten times faster than any other method at that time. The design was first used successfully at the Humber River Sewer Tunnel in 1956, and since then, substantially all modern hard rock tunnel boring machines use rotating cutting wheels with circular disc cutters.
Modern tunnel boring machines use a rotating cutterhead assembly having a plurality of disc-type cutter assemblies rotatably mounted on a front face of the cutterhead. The cutterhead assembly is pushed with great force against the rock face and rotated such that the cutter assemblies loosen, fracture, and/or break up the ground or rock face. The cutterhead assembly may also include other cutting components, for example, scrapers and the like. As the cutterhead is rotated and pressed against the strata, the fractured and loosened material passes through the cutterhead assembly and is deposited onto a conveyor system and transported to the rear of the machine for removal. The modern TBM typically uses a hydraulic gripper system that pushes against the side walls of the tunnel to urge the cutterhead assembly against the rock face, and to propel the TBM forward.
In fractured rock, shielded hard rock TBMs can be used, which erect concrete segments to support unstable tunnel walls behind the machine. Double shield TBMs will generally be operable in two modes, depending on the application. In stable ground, a double shield TBM will grip or react against the tunnel walls to advance the TBM. In unstable, fractured ground, the thrust forces are shifted to thrust cylinders that push off against the tunnel segments behind the machine.
The tunnel size for TBMs typically is in the range of from about a meter in diameter to 19 meters or more. The largest diameter hard rock TBM is believed to be the so-called “Big Becky” manufactured by The Robbins Company to bore a 14.4 meter hydroelectric tunnel beneath Niagara Falls for Canada's Niagara Tunnel Project. Larger TBMs have been constructed for boring through soft ground including sand and clay.
TBMs have the advantage of limiting the disturbance to the surrounding ground (as opposed to conventional drilling and blasting methods), and producing a smooth tunnel wall. In particular, TBMs are often suitable for use even in populated areas. However, the major disadvantage is the large up front costs associated with TBMs. TBMs are expensive machines. The high costs are due, in part, to the fact that a TBM is typically custom designed based on the requirements for a particular project. For example, the power requirements for rotatably driving the cutterhead assembly will depend on aspects of a particular project such as the size of the tunnel, the material to be bored through, and the ground conditions. Such custom design and fabrication requires significant lead times, which can contribute to the critical path for completion of a project. It would be beneficial to improve the TBM to reduce the costs of the machine, to shorten the lead time for production, and to allow for re-use and repurposing of a TBM.